The core of 3C 273 (3C 273 B as it was called in the 1960s) is a very
bright radio source. The presence of the jet described above at large
angular distances showed that the source is not point like, but has an
interesting geometry. Both facts made 3C 273 a prime source for high
resolution radio observations as these became possible by using
together telescopes spanning approximately the size of the Earth (Very
Long Baseline Interferometry, VLBI in short). Early observations used
few telescopes and did not produce maps but were able to measure
whether or not the source is extended at certain angular scales.
[Broten et
al. 1967]
and [Clark et
al. 1967]
showed thus that 3C 273B
has an angular size less than 0.005" at a wavelength of 18cm.
Structure on the scale of milli arcseconds (mas in the following) has
been found by
[Knight et
al. 1971]
and [Cohen et
al. 1971]
who also note
a difference between their results that can be interpreted by a change
in angular size of the source. Radio data (visibility functions rather
than maps) confirmed the reality of the changes and revealed a steady
expansion of the source between 1970 and 1977
[Cohen et
al. 1977].
Study of the location of the minima in the visibility curves showed an
apparent expansion velocity of 5.2c (H0 = 55km/s/Mpc)
[Cohen et
al. 1979].
Maps were also obtained then, showing for the
first time a real jet structure at a position angle of -117°,
not aligned with the larger scale jet described above
[Readhead et
al. 1979].
Subsequent maps at higher angular resolutions
and using more antennae than previously, thus improving the image
quality, showed that the local maximum of the jet moves away from the
core. The distance from the core to the main jet feature had increased
from 6mas in 1977.5 to 8mas in 1980.5
[Pearson et
al. 1981]
corresponding to an apparent expansion velocity of 9.6 ± 0.5c
(H0 = 55km/s/Mpc).

More recent VLBI observations have continued the work done at cm
wavelengths, have used higher frequency observations to increase the
angular resolution and have improved on the dynamical range to study
weaker features. These modern data have confirmed the picture
described above and added several new features.

A set of several VLBI observations in the 1980s has revealed that new
jet components (often called blobs) appear every few years. These
components can be followed from one observation to the next and their
projected trajectories mapped. It is thus possible to trace back each
component to the time of zero separation from the core
[Krichbaum et
al. 1990].
One of the component was observed to be thus
"born" shortly after a violent synchrotron outburst that had been
observed at wavelengths as short as the visible band in March 1988
(see above;
[Courvoisier et
al. 1988]).
This close association suggests
that in general new components in the jet follow synchrotron
outbursts. This is indeed claimed in a study of
[Abraham et
al. 1996]
in which the ejection time of 8 components is computed and
qualitatively compared to single dish light curves. We show in
Fig. 11 the
high frequency radio light curves
and a near infrared light curve available to us (see above) and the
epochs of appearance of new jet components as computed by
[Abraham et
al. 1996].
Whereas it seems clear that the ejection of C9
is associated with the infrared outburst discussed above, no clear
statement can be made for the preceding ejections.

Figure 11. millimetre and infrared light
curves and dates of appearance of new VLBI jet components (see the
text). The components are labeled as in . The epochs of
ejection of the components are from . The
uncertainty in the ejection epochs are shown by a short range.

[Abraham et
al. 1996]
have also correlated the epoch of ejection of components with
the radio light curve at 22GHz they claim that the ejection times of
all components are related to increases in the radio flux. They do
not, however, provide a quantitative assessment of this
relationship. Flux increases are indeed expected to be associated with
the appearance of new jet components if these are new ejecta that
become optically thin as they move away from the core. A further
possible link has been established by
[Krichbaum et
al. 1996]
between the ejection of the knots and the high energy activity of 3C 273 as
evidenced by ECRET data.

The VLBI observations quoted in
[Krichbaum et
al. 1990]
were made at
43GHz. VLBI observations at even higher frequencies (100GHz) were
obtained by
[Bååth et
al. 1991].
These data reach a resolution of
50micro seconds of arc, illustrating the power of the technique.
Using H0 = 50km/s/Mpc this angular resolution
corresponds to a
linear scale of 51017cm at the distance of 3C 273. This is to
be compared with the gravitational radius of a 1010 solar masses
black hole, which is 31015cm. In other words, modern VLBI
observations are capable of resolving structures in the radio data of
3C 273 down to 100 gravitational radii. This effort
to obtain maps at higher frequencies is being pursued (see e.g.
[Krichbaum et
al. 1997]).

High angular resolution VLBI data reveal that the angle at which the jet
emerges
from the core is significantly different at the hundred micro arcsecond
scale (-119°) from that observed at the mili arcsecond scale
(-130°) or at longer scales (-137°)
([Bååth
et al. 1991]
and references therein).
[Bååth et
al. 1991]
interpret this result as being due to either deflection of the jet or
(but this is in a sense equivalent) to changes in the speed of the
jet. This must be put in parallel with the observation of
[Krichbaum et
al. 1990]
who report that the velocities of the
individual knots are different (from 4 ± 0.3 to 8 ± 0.2 times
the velocity of light).

Another type of improvement in the knowledge of the jet was brought about by
investigations with a higher dynamical range. Such observations are
reported in
[Davis Unwin &
Muxlow1991].
Two important results follow from their data. The superluminal motions
observed at small distances from the core
extend to at least 240pc (H0 = 50km/s/Mpc). The
velocity at large distances is only marginally less than closer to the
core. The second result is that no counter
jet is detected. The brightness ratio between a jet and an
intrinsically identical counterjet is given by

Using a spectral index of 0.8
[Davis Unwin &
Muxlow1991]
deduce from the observed lower limit on this ratio that
0.95. This
velocity is close to that obtained from the superluminal expansion
(see below). The data available is therefore still compatible with the
presence of a counter jet of similar properties to the one observed
but unobserved due to its relativistic motion away from us. A further
improvement of the dynamical range by a factor of a few would provide an
important set of data on the intrinsic properties of an eventual
counter jet.

The intrinsic velocity of a relativistic jet can be deduced from the
apparent proper motion in the following way (the original model is due
to [Blandford
McKee & Rees 1977]):

Figure 12. The VLBI Jet of 3C 273 at two
different epochs in 1994 and 1995 observed at
86GHz. (courtesy T. Krichbaum.)

Assume that a photon is emitted by a blob of the jet that has traveled
during t at the
velocity v. The difference in arrival time of
this photon and one that originated from the base of the jet at the
time of departure of the blob
t is

The motion of the blob a perpendicular to the line of sight seen by an
observer far away is

It is easily seen from both expressions that superluminal motion can
be observed for v close to c and
cos close to one and that
the angle for which the
transverse velocity is
maximum for a given intrinsic velocity is given by